Conversion of propylene to ethylene

Abstract

Processes and systems for the conversion of propylene to ethylene may include introducing a propylene feed stream to a C3 metathesis reactor, converting the propylene to ethylene and 2-butene. The metathesis reactor effluent may be recovered and separated in a fractionation system to recover an ethylene product, a C3 fraction, a C4 fraction, and a C5+ fraction. All or a portion of the C3 fraction may be fed to the C3 metathesis reactor to produce additional ethylene. The C4 fraction may be converted in a C4 isomerization/metathesis reaction zone by: (i) isomerization of 2-butenes to 1-butene, (ii) metathesis of the 1-butene and 2-butene to produce propylene and 2-pentene, and/or (iii) autometathesis of the 1-butene to produce ethylene and 3-hexene. An effluent from the C4 isomerization/metathesis reaction zone may then be recovered and fed from the C4 isomerization/metathesis reaction zone to the fractionation system.

What is claimed: 
     
       1. A process for the conversion of propylene to ethylene, comprising:
 introducing a propylene feed stream to a C3 metathesis reactor; 
 contacting the propylene with a metathesis catalyst in the C3 metathesis reactor to convert the propylene to ethylene and 2-butene and recovering an effluent from the C3 metathesis reactor; 
 separating the effluent in a fractionation system to recover an ethylene product fraction, a C3 fraction, a C4 fraction, and a C5+ fraction; 
 feeding all or a portion of the C3 fraction to the C3 metathesis reactor to produce additional ethylene; 
 feeding the C4 fraction to a C4 isomerization/metathesis reaction zone; 
 converting the C4 fraction in the C4 isomerization/metathesis reaction zone by: (i) isomerization of a portion of the 2-butenes to 1-butene, (ii) metathesis of the 1-butene and 2-butene to produce propylene and 2-pentene, and/or (iii) autometathesis of the 1-butene to produce ethylene and 3-hexene; 
 recovering an effluent from the C4 isomerization/metathesis reaction zone, the effluent comprising ethylene, propylene, butenes, pentenes, and hexenes; and 
 feeding the effluent from the C4 isomerization/metathesis reaction zone to the fractionation system. 
 
     
     
       2. The process of  claim 1 , further comprising feeding at least a portion of the C5+ fraction to the C3 metathesis reactor. 
     
     
       3. The process of  claim 2 , wherein the effluent from the C3 metathesis reactor comprise C7+ olefins, the process further comprising purging a portion of the C5+ fraction. 
     
     
       4. The process of  claim 2 , further comprising isomerizing 2-pentene and 3-hexene in the C3 metathesis reaction zone to produce 1-pentene and 1-hexene. 
     
     
       5. The process of  claim 1 , further comprising withdrawing a portion of the C3 fraction as a C3 product fraction. 
     
     
       6. The process of  claim 1 , further comprising feeding a mixed C4 fraction comprising n-butenes to the C4 isomerization/metathesis reaction zone. 
     
     
       7. The process of  claim 1 , wherein the C4 isomerization/metathesis reaction zone comprises an isomerization reactor for isomerizing the 2-butene to 1-butene, a C4 superfractionator for separating a 1-butene fraction from a 2-butene fraction, and a metathesis reactor for converting the 1-butene via autometathesis to ethylene and 3-hexene and for converting 1-butene and 2-butene via metathesis to propylene and 2-pentene. 
     
     
       8. The process of  claim 7 , further comprising feeding at least a portion of the 2-butene fraction recovered in the C4 fractionator to the isomerization reactor. 
     
     
       9. The process of  claim 1 , further comprising:
 feeding hydrogen and a mixed C5 stream to a decyclopentenizer to selectively hydrogenate any cyclopentadiene in the mixed C5 stream and to recover an overheads fraction comprising n-C5 olefins and i-C5 olefins, including 2-pentene, 1-pentene, 3-methyl 1-butene, and 2-methyl-2-butene, and a bottoms stream and/or side draw comprising cyclopentenes; 
 feeding ethylene and the overheads fraction to a C5 isomerization/metathesis reaction zone for:
 isomerizing the 1-pentene to 2-pentene; 
 isomerizing the 3-methyl-1-butene to 2-methyl-2-butene; 
 metathesizing the 2-pentene with ethylene to produce 1-butene and propylene; and 
 metathesizing the 2-methyl-2-butene with ethylene to produce isobutene and propylene; 
 
 recovering the metathesis products and unreacted feed components as an effluent from the C5 isomerization/metathesis reaction zone; 
 fractionating the effluent to recover an ethylene fraction, a propylene fraction, a C4 fraction, and a C5 fraction; 
 feeding the C5 fraction to the C5 isomerization/metathesis reaction zone. 
 
     
     
       10. A system for the conversion of propylene to ethylene, comprising:
 a flow line for providing propylene from a feed source; 
 a C3 metathesis reactor for contacting the propylene with a metathesis catalyst to convert the propylene to ethylene and 2-butene and recovering an effluent from the C3 metathesis reactor; 
 a fractionation system for separating the effluent to recover an ethylene product fraction, a C3 fraction, a C4 fraction, and a C5+ fraction; 
 a flow line for feeding all or a portion of the C3 fraction to the C3 metathesis reactor; 
 a flow line for feeding the C4 fraction to a C4 isomerization/metathesis reaction zone; 
 the C4 isomerization/metathesis reaction zone, for converting the C4 fraction by: (i) isomerization of a portion of the 2-butenes to 1-butene, (ii) metathesis of the 1-butene and 2-butene to produce propylene and 2-pentene, and/or (iii) autometathesis of the 1-butene to produce ethylene and 3-hexene; 
 a flow line for feeding an effluent from the C4 isomerization/metathesis reaction zone to the fractionation system, the effluent comprising ethylene, propylene, butenes, pentenes, and hexenes. 
 
     
     
       11. The system of  claim 10 , further comprising a flow line for feeding at least a portion of the C5+ fraction to the C3 metathesis reactor. 
     
     
       12. The system of  claim 11 , further comprising a flow line for purging a portion of the C5+ fraction. 
     
     
       13. The system of  claim 10 , further comprising a flow line for withdrawing a portion of the C3 fraction as a C3 product fraction. 
     
     
       14. The system of  claim 10 , further comprising a flow line for feeding a mixed C4 fraction comprising n-butenes to the C4 isomerization/metathesis reaction zone. 
     
     
       15. The system of  claim 10 , wherein the C4 isomerization/metathesis reaction zone comprises an isomerization reactor for isomerizing the 2-butene to 1-butene, a C4 superfractionator for separating a 1-butene fraction from a 2-butene fraction, and a metathesis reactor for converting the 1-butene via autometathesis to ethylene and 3-hexene and for converting 1-butene and 2-butene via metathesis to propylene and 2-pentene. 
     
     
       16. The system of  claim 15 , further comprising a flow line for feeding at least a portion of the 2-butene fraction recovered in the C4 fractionator to the isomerization reactor. 
     
     
       17. The system of  claim 10 , further comprising:
 a flow line for providing hydrogen and a flow line for providing a mixed C5 stream; 
 a decyclopentenizer to selectively hydrogenate any cyclopentene and/or cyclopentadiene in the mixed C5 stream and to recover a C5 overheads fraction comprising n-C5 olefins and i-C5 olefins, including 2-pentene, 1-pentene, 3-methyl 1-butene, and 2-methyl-2-butene, and a bottoms stream and/or side draw comprising n-pentanes, i-pentanes, cyclopentene and/or cyclopentane; 
 a C5 isomerization/metathesis reaction zone receiving an ethylene feed and the C5 overheads fraction, the C5 isomerization/metathesis reaction zone being configure for:
 isomerizing the 1-pentene to 2-pentene; 
 isomerizing the 3-methyl-1-butene to 2-methyl-2-butene; 
 metathesizing the 2-pentene with ethylene to produce 1-butene and propylene; and 
 metathesizing the 2-methyl-2-butene with ethylene to produce isobutene and propylene; 
 
 a flow line for recovering an effluent from the C5 isomerization/metathesis reaction zone; 
 a separation system for fractionating the effluent from the C5 isomerization/metathesis reaction zone to recover an ethylene fraction, a propylene fraction, a C4 fraction, and a C5 fraction; 
 a flow line for feeding the C5 fraction to the C5 isomerization/metathesis reaction zone. 
 
     
     
       18. The system of  claim 17 , wherein the ethylene fed to the C5 isomerization/metathesis reaction zone comprises (i) a fresh ethylene feed, (ii) the ethylene fraction, and/or (iii) a portion of the ethylene product fraction. 
     
     
       19. The system of  claim 18 , further comprising a flow line for feeding the propylene fraction to the C3 metathesis reaction zone. 
     
     
       20. The system of  claim 19 , further comprising a catalytic distillation reactor for receiving the C4 fraction and concurrently: (a) isomerizing the 1-butene to 2-butene, (b) separating the 2-butene from the isobutene, (c) recovering the isobutene in an overheads fraction, and (d) recovering the 2-butene in a bottoms fraction. 
     
     
       21. The system of  claim 20 , further comprising a flow line for feeding the bottoms fraction from the catalytic distillation reactor to the C4 isomerizaton/metathesis reaction zone. 
     
     
       22. The system of  claim 21 , further comprising a flow line for feeding a mixed C4 fraction to the catalytic distillation reactor. 
     
     
       23. The system of  claim 19 , further comprising a flow line for feeding the C5+ fraction to the C5 isomerization/metathesis reaction zone.